This invention relates to treatment of large water bodies by circulation. More particularly, this invention relates to means of improving water quality in a surface-frozen, stratified, or otherwise non-uniform water bodies.
Circulation devices are utilized in lakes, ponds, wastewater basins, and other large water bodies for a number of purposes which benefit the public health, recreational and aesthetic value of the water body, as well as downstream waters.
Circulation is useful for prevention of winter kill of fish. Fish in water bodies in cold climates are subject to suffocation when ice and/or ice and snow cover prevents renewal of the dissolved oxygen supply. Dissolved oxygen renewal typically occurs by two means: (1.) direct diffusion from the atmosphere to the exposed open water surface, and (2.) photosynthesis by living plants, using sunlight entering either through the exposed open water surface or through relatively clear ice with little or no snow cover. The invention facilitates this oxygen transfer process by two means: (1.) By bringing warmer, denser bottom water vertically up to the surface, melting ice or ice/snow cover on the surface to create an area of open water in the proximity of the device, exposing water directly to the atmosphere to allow oxygen renewal by direct diffusion and photosynthesis; and (2) By horizontally and vertically circulating to continually distribute oxygen-rich water from the open water area to remote parts of the water body, and replace said oxygen-rich water with oxygen-poor water from remote parts of the water body. Said oxygen-poor water is able to absorb more oxygen than would continuously re-circulated oxygen-rich water, increasing efficiency of oxygen transfer. Horizontal circulation also transports the oxygen-rich water to remote parts of the water body, increasing the overall benefit.
Circulation is used as a means of controlling or reversing stratification. In summer, water bodies, if sufficiently deep, tend to separate into a warmer upper layer known as the epilimnion, a cooler lower layer known as the hypolimnion, and an intermediate transition zone known as the metalimnion. That point in the metalimnion with the steepest thermal gradient is known as the thermocline. The high temperature gradient of the thermocline acts as a barrier against mixing of the hypolimnion and epilimnion. The depth of the epilimnion is thus also called the mixed depth. Vertical circulation through the metalimnion increases the mixed depth and reduces the size of the hypolimnion. If sufficient mixing is provided, stratification can be totally prevented or eliminated. Circulation for partial or complete de-stratification is practiced toward several useful purposes.
Increasing mixed depth through circulation is a means of controlling algae by limiting the amount of light available for photosynthesis at the deeper depths.
Increasing mixed depth through circulation is utilized to increase the size of aerobic habitat. In highly fertile or eutrophic water bodies, decaying dead algae from the epilimnion sinks into the dark hypolimnion, consuming oxygen as it decomposes. Oxygen demand in the lower strata exceeds supply due to (1.) high oxygen demand from settling of dead algae and other organic solids, combined with (2) lack of atmospheric contact for direct diffusion, and lack of light for photosynthesis. In time, the oxygen supply in the hypolimnion becomes depleted. Aerobic animals, including desirable species such as game fish and zooplankton, are forced to crowd into the epilimnion. Zooplankton populations are depleted as they lose their dark refuge, and become easy prey for small planktivorous fish. A deeper epilimnion resulting from circulation provides a larger range for desirable game fish. A deeper epilimnion also results in additional dark refuge for zooplankton, which eat excessive algae.
De-stratification by circulation is a means for prevention of hydrogen sulfide odors originating in the anoxic hypolimnion and sediment-water interface. When dissolved oxygen becomes essentially absent in the lower water strata and upper sediment, anaerobic bacterial action becomes dominant. Anaerobic bacterial action creates hydrogen sulfide gas, forming bubbles which float to the surface and are absorbed into the atmospheric air. Hydrogen sulfide gas produces an offensive odor which can be transmitted by winds over long distances from the water body. De-stratification and circulation of the water body inhibit anaerobic bacterial action by continually exchanging oxygen-depleted water in the lower strata with oxygen-renewed water from the upper strata, where atmospheric contact for direct oxygen diffusion and light for photosynthesis are present.
A poorly circulated water body tends to give a competitive advantage to the undesirable bluegreen algae(cyanophyceae) over the less offensive green algae (chlorophyceae) and diatoms (bicillariophyceae). This is due to two causes: (1) Greens and diatoms have higher sinking rates compared to the more buoyant blue-greens. By remaining closer to the surface, blue-greens have a higher exposure to the light necessary for photosynthesis. (2) Blue-greens have a greater capacity to photosynthesize at lower concentrations of carbon dioxide, a necessary nutrient. De-stratification and circulation of the water body shift the competitive advantage to the greens and diatoms in two ways: (1) Circulation keeps greens and diatoms in suspension close to the surface for a longer period of time, increasing light availability for photosynthesis. (2) De-stratification increases carbon dioxide concentration near the surface, favoring the greens and diatoms. Growth of algae depends on the availability of soluble reactive phosphorous. A major source is phosphorous released from organic sediment in the absence of oxygen. When de-stratification increases oxygen content at the sediment-water interface, release of soluble reactive phosphorous is inhibited and algae growth is reduced.
Circulation will reduce or eliminate stagnation of water bodies and its undesirable effects, including: (1) floating live surface vegetation such as filamentous algae species (spirogyra, cladophora, etc.) and duckweed; (2) floating dead surface vegetation, scum, and oils; (3) odors associated with floating decaying vegetation, and (4) insect reproduction.
Circulation is also a means to facilitate mixing of desirable additives. For example, phosphorous-rich water may be mixed with an agent such as aluminum or iron salts, which combine with soluble reactive phosphorous to form a precipitate which settles to the bottom. Circulation can also distribute algaecides, lime for neutralization of acid rain, or other chemical agents.
Certain water bodies, particularly treated wastewater effluents, are high in nitrate. Circulation in the absence of oxygen facilitates de-nitrification, the process by which nitrate is reduced to molecular nitrogen by facultative anaerobic bacteria.
De-stratification and circulation devices presently in use generally are of one of several types. One class of devices is fountains driven by impeller pumps, such as shown in U.S. Pat. No. 3,865,909 (Cramer). Fountains pump water from below the surface and spray into the air, usually in a circular pattern. A disadvantage of fountains is that discharged water tends to re-circulate back through the fountain, so that only a small area is circulated, with little or no effect on remote parts of the water body. A further disadvantage of fountains is high energy consumption, as excessive kinetic energy is concentrated in a small volume of water. A further disadvantage of fountains is that they are exposed to inclement weather, and can be damaged by high waves, floating ice masses, ice formation, and temperature extremes. A further disadvantage of fountains is that they are a hazard to navigation.
De-stratification and circulation is also achieved by air bubblers, whereby a compressor pumps air through a pipe or hose to be released below the water surface, either directly or through a diffusion device. Bubbles are formed, creating a buoyant air-water mixture which rises to the surface. A disadvantage of air bubblers is that water tends to re-circulate back through the air bubblers, so that only a small area is circulated, with little or no effect on remote parts of the water body. A further disadvantage of air bubblers is high energy consumption, as overall circulation efficiency of an air-lift device is poor. A further disadvantage of air bubblers is the tendency of air pipes or hoses to float to the surface, where they may be subject to damage by ice or boats. A further disadvantage of air bubblers is that air lines and diffusion devices can leak or become clogged by ice or debris.
De-stratification and circulation is also achieved by surface propeller or paddle wheel devices, whereby an impeller, driven by a prime mover, is suspended at the water surface from a float or support structure. Many embodiments are known. The impeller is aimed and rotated to produce circulation in a desired direction. Some embodiments, such as shown in U.S. Pat. No. 4,774,031, are used with air injection, whereby bubbles aid the circulation by creating a buoyant air-water mixture which rises to the surface. In another embodiment, illustrated by U.S. Pat. No. 4,764,313, an impeller is driven by a vertical axis wind turbine, all on an anchored floating structure. A disadvantage of surface propeller or paddle devices is that their influence is limited to waters near the surface. A further disadvantage of surface propeller or paddle wheel devices is that they are exposed to inclement weather, and can be damaged by high waves, floating ice masses, ice formation, and temperature extremes. A further disadvantage of surface propeller or paddle wheel devices is that they are a hazard to navigation.
De-stratification and circulation is also achieved by surface-retrievable submersible propeller devices. One embodiment is illustrated by U.S. Pat. No. 5,338,116 (Spurl). Circulation is produced by a propeller rotated by a submersible electric motor, and the device is supported by a track structure which protrudes up through the water surface, enabling the device to be raised to the surface for service or replacement. A disadvantage of surface-retrievable submersible propeller devices is that their structure which protrudes the surface is exposed to inclement weather, and can be damaged by high waves, floating ice masses, ice formation, and temperature extremes. A further disadvantage of surface-retrievable submersible propeller devices is that stationary surface-penetrating support tracks or cables tend to collect floating debris such as dead weeds, rags, and plastic bags. A further disadvantage of surface-retrievable submersible propeller devices is that their structure which protrudes the surface is a hazard to navigation.
In summary, known apparatuses are limited in application due to: (1.) Limitation of the horizontal and vertical range of influence; (2.) High energy consumption; (3.) Susceptibility to damage or destruction due to natural elements such as high waves, wind, floating ice masses, ice formation, and temperature extremes; (4) Susceptibility to damage or destruction from human activities such as vandalism; (4) Susceptibility to collect floating debris; and (5.) Navigational hazard.